The present invention is concerned with neutralising space charge in ion beams.
Ion beams may be used for a number of purposes. In particular, ions of desired dopant species can be implanted in semiconductor substrates. So called xe2x80x9cion implantersxe2x80x9d produce a beam of ions of the required dopant species which is directed at the substrate to implant the ions into the semiconductor material.
In the absence of any neutralising effect, a beam containing only ions of a particular polarity will is experience space charge effects. The mutual repulsion of the ions in the beam tends to cause the beam to xe2x80x9cblow upxe2x80x9d and become uncontrollable. In regions of zero electric field, self neutralisation of ion beams tends to occur, through the production of electrons resulting from collisions between beam ions and slow moving atoms of residual gas in the vacuum chamber through which the beam is passing. However, in regions of electric field, for example when the ions of the beam are being accelerated or decelerated, or the beam is being electrostatically deflected, neutralising electrons are quickly removed from the beam because of their high mobility.
It is therefore a problem to control ion beams and prevent beam blow up, especially in regions containing an external electric field. The effect of beam space charge is especially severe for relatively low energy beams, since for the same beam current, there is a higher density of ions in a low energy beam.
In machines, such as ion implanters, where it is required to present a beam having a predetermined energy onto a target, particular problems arise where the ion beam is first extracted from the ion source, and elsewhere over the beam path where the beam energy is changed, particularly where the beam is decelerated prior to hitting the target. In the case of extraction of ions from an ion source, the need to obtain a usefully high beam current limits the minimum energy at which ions can be extracted from the arc chamber of the source. The region between the exit window of the source arc chamber and the extraction electrode is a region of substantial electric field in which electrons cannot exist for significant times. In the extreme, for high currents and low extraction energies, the theoretical density of ions in the beam leaving the source produces a beam potential similar to the potential on the extraction electrode. Clearly in such a case, the ions in the beam will not experience the required accelerating field and no beam is in fact produced. In fact, space charge effects can severely reduce the extraction efficiency from ion sources at low extraction energies.
In ion implanters, it has therefore been the usual practice to extract ions from the ion source at no less than a minimum extraction energy, typically not less than 10 keV, and for good efficiency often at higher energies. In order to present a beam of lower energy onto the target substrate, the beam must be subsequently decelerated, and space charge problems then arise also in the region of the decelerating field.
There is accordingly a requirement for some mechanism for the efficient extraction of ions from an ion source at relatively low energies, and more generally for arrangements for controlling and neutralising space charge effects in ion beams, especially in regions of externally applied electric field.
According to the present invention, a method of neutralising space charge in an ion beam comprising ions of a first polarity, comprises the steps of generating ions of a second polarity, and introducing said second polarity ions to space charge neutralise the ion beam.
The ions of second polarity should be ions of species which can be tolerated in the ion beam process. For example in an ion implantation process the ions may be of atoms such as He, Ne, Ar, Kr or Xe, or of molecules such as N2, CO2, CO2, CO, O2, Cl2, Br2, I2. In each instance, the selection of an appropriate species is dependent on the need to avoid unwanted reactions and effects in the beam process. For example O2 would not be suitable for neutralising an ion beam being extracted from an ion source employing a hot cathode, such as a Bernas source, as the O2 would quickly corrode the cathode. However O2 may be tolerated with R.F. or microwave energised ion sources.
For effective space charge neutralisation, the density of ions of the first polarity in a region of the ion beam should equal the density of charged particles of the opposite polarity, assuming both the ions and particles are singly charged. However, the density of charged particles in a beam is proportional to the current density of those particles in the beam and inversely proportional to the velocity of the particles in the beam direction. The velocity of charged particles, on the other hand, is proportional to the square root of the energy of the particles and inversely proportional to the square root of the mass of the particles.
The overall effect is that singly charged particles of high mass will have much lower velocities, for the same energy, than lighter particles and especially when compared with electrons. Thus, beam space charge can be effectively neutralised with opposite polarity ions in regions of the beam where neutralisation by electrons may not be possible.
In order to reduce the current density of the second polarity ions in the beam, in a region of applied electric field, required to maintain neutralisation, the mass of the second polarity ions should be as high as possible. Large molecules, including organic molecules may then be useful, such as B10H12, CxHy (hydro-carbons) or CxHyOH (alcohols). Alternatively large cluster ions may be employed.
Preferably, the second polarity ions should have a mass/charge ratio of at least 400.
In a preferred example, the method includes applying an external electric field to a region of the ion beam and introducing said second polarity ions in said region. As explained previously, beam neutralisation problems arise especially in regions of external electric field.
Then said second polarity ions are accelerated by said external electric field in a field gradient direction, and said second polarity ions may be introduced at a location in said region which is upstream relative to said field gradient direction.
It is normal practice in ion beam machines where a positive ion beam is accelerated or decelerated between a first electrode at a first potential and a second electrode at a second higher potential, to provide an electron suppression electrode between the first and second electrodes at a potential which is more negative than that of the first electrode, to prevent electrons being drawn from the beam beyond the first electrode by the influence of the potential on the second electrode. Then, for such a positive ion beam, ions of negative polarity are preferably introduced adjacent to the electron suppression electrode.
The invention also provides apparatus for neutralising space charge in an ion beam, comprising means producing a beam of ions of a first polarity, means generating ions of a second polarity, and means introducing said second polarity ions to space charge neutralise the ion beam.